In general, a fluid pressure device supplies a working medium such as working oil and the like into a pressure chamber formed by a cylinder and a piston axially movably mounted with this cylinder with a predetermined pressure or increases the pressure by applying a load to the working medium sealed into the pressure chamber, thereby converting the energy of the working medium into the mechanical energy, that is, a driving force of the piston. To prevent a leakage of oil pressure from the pressure chamber, a seal member is mounted on the sliding portion with the cylinder and the piston.
Meanwhile, heretofore, there has been frequently used a fastening method by a bolt and a nut in case a turbine case of a steam turbine or a gas turbine, which is used in the power plant of an electric power station and the like, is assembled. However, in such a fastening method, since almost all fastening torque applied to a nut oppose the rolling friction in a bearing surface, it is difficult to obtain a high fastening force by increasing an axial pressure of the bolt. Hence, in case the high fastening force is required in this manner, the fluid pressure device is incorporated, whereby there is used a fluid pressure nut which is screw-fastened in a state in which a bolt is applied with a tensile force in advance.
The fluid pressure nut comprises a circular cylinder disposed on the turbine case and a piston axially movably mounted with the cylinder and, between the cylinder and the piston, there is defined a pressure chamber. The axial center of the piston is formed with a screw hole, and this screw hole is screw-fastened on a bolt inserted into the insertion hole of the turbine case. Further, the fluid pressure nut is provided with a lock ring, and this lock ring is screw-fastened on an outer peripheral surface of the cylinder so as to be movable in a direction of approaching or being isolated from the end portion of the piston. When the oil pressure is supplied to the pressure chamber in a state in which the piston is screw-fastened on the bolt, the piston moves in a direction of being isolated from the cylinder, and applies a load in a tensile-force direction to the bolt. After that, the lock ring is rotated until a position to contact the end portion of the piston and when the oil pressure inside the pressure chamber is evacuated, the load applied to the piston is put into a state supported by the lock ring, thereby completing the fastening. In this manner, since the fluid pressure nut is screw-fastened in a state in which the bolt is applied with a tensile force, this increases an axial tension of the bolt, thereby making it possible to obtain a high fastening force.
In such a fluid pressure nut, to prevent the working medium such as the working oil and water from leaking from the pressure chamber, the end portion of the pressure chamber, that is, the sliding portion with the cylinder and the piston is provided with a seal member. Conventionally, as such a seal member, an O-ring, U-packing, X-ring, and the like are frequently used, and in this case, as the material thereof, elastic materials such as rubber, plastic, and the like are used. Even in case a gap of the sliding member increases due to pressure deflection, in conformity with the change of this gap, the seal member is elastically deformed and maintains the sealing.
However, since the turbine case has a plurality of bolts lined up and arranged with a small pitch, there are often the cases where the fluid pressure nut is limited in its outer diameter size and is unable to set a pressure receiving area of the piston large. Hence, to apply a predetermined tensile force to the bolt, the oil pressure supplied inside the pressure chamber must be increased and, in some cases, it becomes a super high pressure of 250 Mpa or more. In such a case, the seal member is unable to endure the super high pressure, and it has been difficult to prevent the leakage of the oil pressure.
In contrast to this, there has been known a turbine case in which the outer peripheral end of the piston is formed in a knife edge shape which closely contacts the cylinder or mounted with a U-cup type seal ring made of rigid plastic or metal at the end of the pressure chamber.
However, since the turbine case reaches extremely high temperature by high temperature and pressure gas involved, the fluid pressure nut fastened is exposed to high temperature environment that exceeds 500 degree. Hence, according to the sealing method using the edge portion as described above, there are often the cases where the edge portion is unable to endure the heat and is deformed, thereby lowering a sealing property. In this case, even when the oil pressure is supplied to the interior of the pressure chamber again to take out the fluid pressure nut, the pressure in the interior of the pressure chamber cannot be increased to a rated value and, therefore, taking out this fluid pressure nut is made difficult. Further, in case the pressure of the pressure chamber cannot be increased, although a step such as cutting the lock ring and the like is required, cutting only the lock ring has been not easy.
Further, on the occasion of screw-fastening the bolt and the fluid pressure nut, there are often the cases where the bolt slightly inclines. In this case, according to the turbine case in which a knife edge portion is formed on the outer peripheral end of the piston, the knife edge portion has been deformed due to inclination of the bolt, thereby lowering the sealing property.
An object of the present invention is to raise the upper limit of the oil pressure supplied to the liquid pressure device.
Another object of the present invention is to improve durability of the liquid pressure device under a high temperature environment.